You must not / should not /ought not forget my birthday.
which one is correct?
All 3 are correct. You must not forget my birthday . This is a strong command.
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All 3 are correct.
You must not forget my birthday. This is a strong command.
You should not forget my birthday. This is less strong.
You ought not to forget my birthday. This is also less strong, and in addition is less commonly said.